1. Field of the Invention
The present invention relates to a method for molding an end of a long resin molded article, and particularly to an end molding method in which a front surface of an end portion of a long resin molded article such as a side molding for an automobile, or the like, formed into a general sectional shape by extrusion molding is heated so as to be softened and further pressed into a predetermined shape such as an inclination shape toward a rear surface, or the like.
2. Description of the Related Art
A conventional example of this type method for molding an end of a long resin molded article is a technique shown in FIGS. 15 and 16(a) to 16(c). FIG. 15 is an explanatory view showing a heating step in a conventional method for molding an end of a long resin molded article. FIGS. 16(a) to 16(c) are explanatory views showing a pressing step in the conventional method for molding an end of a long resin molded article. FIG. 16(a) shows a state in which a long resin is disposed in press molds; FIG. 16(b) shows a state in which the end portion of the long resin is pressed by the press molds; and FIG. 16(c) shows a state in which rebounding occurs in an end portion of a long resin molded article obtained by pressing the long resin after evaluation of heat resistance.
This technique is provided to obtain a desired long resin molded article, for example, a finished product such as a side molding for an automobile, or the like, by a molding method in which an end portion of a long resin is heated so as to be softened and further pressed into a predetermined shape, that is, by a so-called hot press method. As shown in FIG. 15, in this technique, an end portion 2 (a left end portion in FIG. 15) of a long resin 1 obtained by extrusion-molding a resin such as polyvinyl chloride (PVC), polyolefins such as polypropylene (PP), polyethylene or the like, thermoplastic elastomer (TPO), vinyl acetate or the like and then cutting the end portion of the long resin obliquely at a predetermined position is concentratedly heated so as to be softened between electrodes 11 and 12 of a high frequency induction heating machine. Then, as shown in FIGS. 16(a) to 16(c), the end portion 2 of the long resin 1 which is softened by heating is pressed to form a predetermined sectional shape in which the end portion 2 is inclined from the front surface side to the rear surface side (from the upper side to the lower side in FIGS. 16(a) to 16(c)), so that a long resin molded article having a predetermined end shape can be obtained. Hereupon, the front surface serves as a design surface which is exposed outside, and the rear surface serves as an attachment surface which is attached to other parts such as an automobile door.
To perform such hot press molding, there are conventionally used press molds composed of an upper mold 15 having a press surface 15a corresponding to a design surface of the long resin molded article, and a lower mold 16 having a press surface 16a corresponding to the rear surface of the long resin molded article. After the steps of extrusion molding a resin, cutting the resin into a predetermined length and heating the end portion of the cut resin, the heated end-portion 2 of the long resin 1 is pressed while being held between the press surfaces 15a and 16a of the upper and lower molds 15 and 16 respectively to there by obtain a predetermined end shape as described above. Incidentally, escape cavities 15b and 16b are formed in the upper and lower molds 15 and 16 respectively so as to serve as portions for making a surplus resin R escape after hot pressing the end portion 2 of the long resin 1, so that the surplus resin R is reserved in the escape cavities so that such surplus resin R never exerts influence on the quality of external appearance of the final product.
Because the conventional method for molding an end of a long resin molded article is configured as described above, residual stress is generated in the inside of the end portion 2 with the plastic deformation of the end portion 2 of the long resin 1 by hot pressing. For example, as shown in FIG. 16(c), there is the possibility that disadvantages such as rebounding of the end portion 2A in a direction (upward in the drawing) reverse to the pressing direction, or the like, may occur after evaluation of heat resistance of the long resin molded article 1A.
Furthermore, conventionally, an insert of a metal such as aluminum, or the like, may be longitudinally embedded in the inside of the long resin molded article such as a side molding, or the like, for the purpose of increasing stiffness, or the like. When the end portion 6 (the right end portion in the drawing) of the long resin 5 having such an insert 7 embedded therein is to be molded into a predetermined shape by a hot press method as shown in FIGS. 17 (a) and 17 (b), the end portion 6 is first concentratedly heated by a high frequency induction heating machine as shown in FIG. 15. In this case, as shown in FIG. 17(b) , there is, however, the possibility that the end portion 6 of the long resin 5 is melted and contracted by high frequency induction heating so that the insert 7 may be exposed from an end surface of the long resin 5 by heating. In this case, sparks may be generated between the electrodes 11, 12 of the high frequency induction heating machine and the insert 7 by high frequency induction heating, so that there is the possibility that pressing operation after that may be made difficult because the high frequency induction heating machine may stop before the temperature of the end portion 6 of the long resin 5 reaches a press-enabled point.
It is an object of the present invention to provide a method for molding an end of a long resin molded article, in which disadvantages such as rebounding of an end portion, or the like, are prevented in evaluation of heat resistance, or the like, after molding.
It is another object of the present invention to provide a method for molding an end of a long resin molded article, in which a heating operation can be performed smoothly even in the case where an end portion of a long resin having an insert embedded therein is heated by high frequency induction heating.
The inventors of the present invention have made studies to find the reasons of the rebounding as shown in FIG. 16(c), and it the following facts were found:
(i) In the hot pressing step as shown in FIGS. 16(a) and 16(b), the end portion of the long resin molded article becomes inclined from the front surface to the rear surface. At this time, the resin of the front surface side is stretched largely by plastic deformation. On the other hand, the resin of the rear surface side is stretched very slightly. During the processing step, the resin of the front surface is plastically deformed to a greater degree than the resin of the rear surface. This difference in plastic deformation results in a residual stress F in the inside of the end portion as shown in FIG. 18. Thus generated residual stress is remained also after the hot pressing, and the residual stress is released by the gradual cooling after the evaluation of heat resistance as shown in FIG. 18. This stress release generates rebounding of the end portion.
(ii) Thermal shrinkage of resin in the thick portion of the end portion (area A in FIG. 18) is larger than that in the thin portion of the end portion (area B in FIG. 18), so that residual stress F shown in FIG. 18 is generated as similar to the above (i). It should be noted that if only at least one of the above factors (i) and (ii) is solved, it becomes possible to achieve the objects of the present invention. Although the rebounding of the end portion is based on the combination of the above (i) and (ii), if one of (i) and (ii) is solved, almost all of the other is also removed.
In a first aspect of the invention, a method is provided for molding an end of a long resin molded article, the long resin molded article having been obtained by extrusion molding and having a design surface and an attachment surface extended in a longitudinal direction of the long resin molded article, the attachment surface being attached to other parts, the method comprising the steps of: heating an end portion of the long resin molded article so as to soften the end portion of the long resin molded article; pressing the design surface of the heated end portion so as to stretch resin on a side of the design surface thereby forming an inclined surface on the end portion, whereby the end portion is inclined from the design surface at the end portion to the attachment surface at the end portion along the inclined surface; and pressing the attachment surface of the heated end portion so as to form a cavity on the attachment surface and stretch resin on a side of the attachment surface, wherein the resin on the side of the design surface is stretched to a substantially equal degree to the resin on the side of the attachment surface.
In a second aspect of the invention, a method is provided for molding an end of a long resin molded article, the long resin molded article having been obtained by extrusion molding and having a design surface and an attachment surface extended in a longitudinal direction of the long resin molded article, the attachment surface being attached to other parts, the method comprising the steps of: heating an end portion of the long resin molded article so as to soften the end portion of the long resin molded article; embedding a piece having a convex surface in the attachment surface of the heated end portion while forming a cavity on the attachment surface so as to bond the convex surface of the piece to the cavity; and pressing the design surface of the heated end portion thereby forming an inclined surface on the end portion, whereby the end portion is inclined from the design surface at the end portion to the attachment surface at the end portion along the inclined surface, wherein a shape of the inclined surface is substantially same as a shape of the convex surface of the piece. In a third aspect of the invention, a method is provided for molding an end of a long resin molded article, the long resin molded article having been obtained by extrusion molding and having a design surface and an attachment surface extended in a longitudinal direction of the long resin molded article, the attachment surface being attached to the parts, the method comprising the steps of: forming a lightened portion at least in the attachment surface of an end portion of the long resin molded article so that the lightened portion extends in a longitudinal direction of the long resin molded article; heating the end portion of the long resin molded article so as to soften the end portion of the long resin molded article; and pressing the heated end portion of the long resin molded article so that the end portion is inclined from the design surface to the attachment surface thereof.
In a fourth aspect of the invention, a method is provided for molding an end of a long resin molded article comprising the steps of: extrusion molding the ling resin molded article having a metal insert longitudinally continuously embedded; cutting the long resin molded article into a predetermined length; sealing a portion of an end portion of the cut long resin molded article corresponding to an end portion of the insert with a sealing material; heating the end portion of the long resin molded article; and pressing the heated end portion of the long resin molded article to a predetermined shape. The invention has been described in Japanese Application No. 8-77574, filed Mar. 29, 1996, the entire contents of which are incorporated herein by reference.